Do you have a Chromecast or an Android TV device? Then open up Chrome on your desktop, make sure that you've got the Chromecast extension installed, go to this website, click "redeem all offers," then click the Google Play offer in the upper-left corner. You'll get a free movie rental voucher on your account, which can be applied when you checkout any movie on the Play Store. HD movies are valid as well. Even though the freebie is specifically to promote the Chromecast's second birthday, connecting to an Android TV box will get you the credit too.

This offer is valid in the United States and a few other countries (commenters mentioned Germany, Australia, and France) but doesn't appear to be a worldwide promotion. Some users are also reporting that they've "redeemed the maximum number of free rentals" after participating in previous freebies, so there appears to be some kind of hard limit.